1. Field of the Invention
The present invention relates to an automatic equalizer which uses a transversal filter.
2. Description of the Prior Art
A device which eliminates distortion components in electric signals by controlling the tap gains of a transversal filter is called an automatic equalizer and is widely used as waveform equalizers and echo cancellers in signal transmission paths. Recently, an attempt is being made to use this automatic equalizer for eliminating ghost or multipath signals in television receivers.
FIG. 1 schematically shows an automatic equalizer which uses a transversal filter. From a terminal 1, an input signal X(t) to be equalized is supplied to a transversal filter 2. A subtracter 3 produces the difference between this input signal X(t) and the output signal of transversal filter 2. By appropriately controlling the tap gains of transversal filter 2, a signal from which distortion components are eliminated, that is, an equalized signal, is obtained at the output of subtracter 3. This output signal Y(t) is applied to a utilization circuit through a terminal 4. When the output signal of the transversal filter 2 is opposite in phase to the input signal X(t), the subtracter 3 may be replaced by an adder.
FIGS. 2A and 2B show practical examples of the transversal filter 2 which is comprised of a tapped-delay-line 21 which consists of an electric charge transfer device such as a CCD (charge coupled device), and weighting circuits 22. The transversal filter shown in FIG. 2A is of the type called an input-weighted transversal filter wherein an input signal is multiplied by weighting coefficients (tap gain) in weighting circuits 22 and the multiplied input signals are applied to corresponding taps of the tapped-delay-line 21. The transversal filter shown in FIG. 2B is of the type called an output-weighted transversal filter wherein an output signal at each tap of the tapped-delay-line is multiplied by a weighting coefficient (tap gain) in a corresponding weighting circuit 22 and the multiplied output signals are synthesized by an adder 23 into an output signal.
Referring again to FIG. 1, the tap gains of the transversal filter 2 are held by a tap gain hold circuit 7. These tap gains are automatically corrected in the following manner. Part of the equalized output signal Y(t) is supplied to a distortion detect circuit 5. The distortion detect circuit 5 detects distortion components which are associated with a reference signal included in the output signal Y(t) or a signal obtained by differentiating the signal Y(t) (vertical synchronizing signals included in video signals in the case of eliminating ghosts). As another example, the distortion detect circuit 5 detects the distortion components in the output signal Y(t) by correlating the input signal X(t) to the output signal Y(t) or the input signal X(t) to a difference between the output signal Y(t) and a reference signal R(t). A detection signal D(t) derived from the distortion ciruit 5 is supplied to a tap gain correct circuit 6. The tap gain correct circuit 6 corrects the tap gain held by the tap gain holding circuit 7 in response to the detect signal D(t). Namely, the circuit 5 periodically judges the polarity of the detect signal D(t) to produce correct signals M(t) which are applied to the tap gain hold circuit 7. The tap gain hold circuit 7 has, for example, a digital memory for storing tap gain information, and the information stored therein is corrected by the correct signals M(t). An output signal of this memory is D/A converted and is supplied to a corresponding weighting circuit 22 of the transversal filter 2 for correcting the corresponding tap gain.
One of the problems with such an automatic equalizer as described above is an influence due to a direct current (DC) offset component which appears at the outputs of the distortion detect circuit 5 and the tap gain correct circuit 6. For example, when there is an echo (ghost) signal as a distortion component, the detect signal D(t) may have such a waveform as shown in FIG. 3A. In the figure .epsilon. represents the DC offset component. Such a DC offset component .epsilon. is regarded as distortion to be eliminated in the system of FIG. 1. Therefore, -.epsilon. is superposed on every tap gain. Consequently, the input signal or the output signal at each tap of the transversal filter 2 is multiplied by -.epsilon.. As a result, a sag of value N .epsilon. is generated in the output signal Y(t), as shown in FIG. 3B, for a rectangular wave input signal. N is the number of taps of the transversal filter 2. Since the delay time of the tapped-delay-line 21 is required to be 20-30 .mu.sec in the case of a television ghost eliminating device, N becomes 200-300. In such a case, for reducing the sag value N .epsilon. to -40 dB, the DC offset component .epsilon. must be reduced below -90 dB. However, it it extremely difficult to reduce the DC offset to such a value. Consequently, waveform distortions such as the generation of sag has been unavoidable in the output signal Y(t).